64 PHYSIOLOGY 



in the manner of the compounding of them. Many illus- 

 trations arise. Carbon compounds preponderate not 

 only in plants and animals but in many rocks and soils. 

 Those of the latter dissolve and from their solvents 

 can be obtained as crystals. Crystalline form is some 

 guarantee of chemical purity, and helps the chemist in 

 separating compounds. Now though the body con- 

 sists of water to the extent of more than half its weight, 

 its carbon-compounds are for the most part not dissolved 

 but exist in a kind of particulate suspension. Unlike 

 those of the rock, they will not crystallize out ; and they 

 contain not definite but variable proportions of water. 

 They do not carry electrical charges in the same way, 

 they do not exert the same pressures or exhibit the 

 same tensions. They offer much greater difficulty to 

 present methods of chemical research. They are in a 

 state which is termed colloidal. Not that the living 

 body is devoid of those simpler chemical compounds 

 which are characteristic of the inanimate world. The 

 living body contains colloids mixed with dilute solutions 

 of crystalloids. And this commingling renders the matter 

 still more intricate. 



How largely the working of the body is a chemical 

 problem is signalized by the vast effect upon it of minute 

 quantities of certain substances acting chemically within 

 it. A few drops of cobra-venom or of a solution of 

 diphtheria poison, introduced through a hollow fang or 

 a hollow needle, and the whole machinery perishes in 

 a few hours or days. Such an effect emphasizes the 

 delicacy of adjustment of the chemistry of life. We can 

 conceive that the body, chemical machine as it is, may 

 sometimes poison itself in its own working. Fatigue 

 from muscular exertion seems such a case : poisons are 

 produced from the superactive muscles more quickly 



